In recent years, as electronic appliances have been made more inexpensive, compact, efficient and energy saving, more compact and efficient switching power supplies have been strongly demanded. Furthermore, for the reason such as the reduction of voltages in integrated circuits, the power supply voltages necessary for the electric appliances have been increasingly reduced. Even as to the switching power supplies conforming to such low power supply voltages, there is a problem that, with a rectifier circuit using usual rectifier diodes, a rectification loss becomes greater relative to the power supply output, and power supply efficiency is thereby reduced.
In recent years, since switching devices such as MOSFETs have become higher in performance, an attempt to configure a switching power supply on the synchronous rectification system where a rectifier circuit comprises such switching devices has been made. The MOSFET has a characteristic that, relative to the same-class rectifier diode, forward drop voltages can be made smaller and the rectification losses can be lower. However, the MOSFET has to be driven in synchronism with the switching power supply, so that an appropriate gate voltage needs to be formed.
A conventional switching power supply on the synchronous rectification system will be described below. FIG. 9 is a circuit diagram showing a configuration of a conventional full-bridge-type switching power supply. In FIG. 9, an input DC power supply 101, which comprises a circuit for rectifying and smoothing a commercial power supply or a battery, is connected across input terminals 102a and 102b. A first switching device 103 and a second switching device 104 each comprise a MOSFET, and the series circuit of the first switching device 103 and the second switching device 104 is connected across the input terminals 102a and 102b. The first switching device 103 and the second switching device 104 are alternately ON/OFF-driven by after-mentioned driving circuits. Similarly, a third switching device 105 and a fourth switching device 106 each comprise a MOSFET, and the series circuit of the third switching device 105 and the fourth switching device 106 is connected across the input terminals 102a and 102b. Furthermore, the third switching device 105 and the fourth switching device 106 are alternately ON/OFF-driven by after-mentioned driving circuits.
A transformer 107 has a primary winding 107a, a first secondary winding 107b and a second secondary winding 107c. One end of the primary winding 107a is connected to the connection point of the first switching device 103 and the second switching device 104, while the other end of the primary winding 107a is connected to the connection point of the third switching device 105 and the fourth switching device 106. The first secondary winding 107b and the second secondary winding 107c of the transformer 107 are connected in series, and the connection point of the first secondary winding 107b and the second secondary winding 107c of the transformer 107 is connected to one end of an inductance device 110.
A first synchronous rectifier device 108 and a second synchronous rectifier device 109 each comprise a MOSFET. Source terminals of the first synchronous rectifier device 108 and the second synchronous rectifier device 109 are connected to each other. The drain terminal of the first synchronous rectifier device 108 is connected to the second secondary winding 107c, while the drain terminal of the second synchronous rectifier device 109 is connected to the first secondary winding 107b. 
One end of the series circuit of the inductance device 110 and a smoothing capacitor 111 is connected to the connection point of the source terminals of, respectively, the first synchronous rectifier device 108 and the second synchronous rectifier device 109. The other end of the series circuit thereof is connected to the connection point of the first secondary winding 107b and the second secondary winding 107c of the transformer 107. A smoothing circuit is comprised of the inductance device 110 and the smoothing capacitor 111.
Output terminals 112a and 112b which output stable electric power are provided across the smoothing capacitor 111. A load 113 which consumes electric power is connected across the output terminals 112a and 112b. 
An auxiliary power supply 115 is connected to the input terminals 102a and 102b, and a stable voltage is supplied from the input DC power supply 101 to this auxiliary power supply 115. A control circuit 114 connected to the auxiliary power supply 115 generates a PWM signal according to a feedback signal from a photocoupler 154.
A first driving circuit 155 to a fourth driving circuit 158 connected to, respectively, the first switching device 103 to the fourth switching device 106 output drive signals of the first switching device 103 to the fourth switching device 106 respectively according to the PWM signals output from the control circuit 114. A first npn transistor 116, a first pnp transistor 117, a second npn transistor 118 and a second pnp transistor 119 are each drive-controlled by the PWM signal from the control circuit 114. A fifth driving circuit 159 comprises the first npn transistor 116 and the first pnp transistor 117, while a sixth driving circuit 160 comprises the second npn transistor 118 and the second pnp transistor 119. The series circuit of a first capacitor 120 and a primary winding 121a of a drive transformer 121 is connected across the fifth driving circuit 159 and the sixth driving circuit 160. The drive transformer 121 has the primary winding 121a, a first secondary winding 121b and a second secondary winding 121c, and is driven according to the PWM signal of the control circuit 114.
A first reverse comprising a first FET 122 and a first resistance 123 is provided on the secondary side of the drive transformer 121 to invert a signal of the first secondary winding 121b of the drive transformer 121. A seventh driving circuit 161 comprises a third npn transistor 124 and a third pnp transistor 125, and drives the first synchronous rectifier device 108 according to the output of the first reverse.
Furthermore, a second reverse comprising a second FET 126 and a second resistance 127 is provided on the secondary side of the drive transformer 121 to invert a signal of the second secondary winding 121c of the drive transformer 121. An eighth driving circuit 162 comprises a fourth npn transistor 128 and a fourth pnp transistor 129, and drives the second synchronous rectifier device 109 according to the output of the second reverse.
In an output voltage detection circuit 350, a first detection resistance 150 and a second detection resistance 151 are connected in series with the output terminals 112a and 112b, and divide an output voltage. A differential amplifier 153 compares the voltage obtained by dividing the output voltage and the voltage of a reference power supply 152 and amplifies the differential therebetween. A limit resistance 163 determines a current to be passed through the photocoupler 154 according to the voltage error-amplified in the differential amplifier 153.
The first reverse, the second reverse, the photocoupler 154, the seventh driving circuit 161 and the eighth driving circuit 162 are connected to the output terminals 112a and 112b, which serve as the drive power supplies thereof.
FIG. 10 is an operation waveform diagram showing a state of the operation of each part in the conventional switching power supply in FIG. 9. A signal VG1 shown in part (a) of FIG. 10 is a first PWM signal of the control circuit 114, and ON/OFF-drives the first switching device 103 and the fourth switching device 106 simultaneously via the first driving circuit 155 and the fourth driving circuit 158. A signal VG2 shown in part (b) of FIG. 10 is a second PWM signal of the control circuit 114, and ON/OFF-drives the second switching device 104 and the third switching device 105 simultaneously via the second driving circuit 156 and the third driving circuit 157.
A signal VT1 shown in part (c) of FIG. 10 represents a waveform of the voltage generated in the first secondary winding 121b of the drive transformer 121. A signal VT2 shown in part (d) of FIG. 10 represents the voltage generated in the second secondary winding 121c of the drive transformer 121. A signal VG3 shown in part (e) of FIG. 10 represents the output of the first reverse. The first synchronous rectifier device 108 is ON/OFF-driven under the signal VG3 from this first reverse. A signal VG4 shown in part (f) of FIG. 10 represents the output of the second reverse. The second synchronous rectifier device 109 is ON/OFF-driven under the signal VG4 from this second reverse.
Furthermore, IG1 shown in part (g) of FIG. 10 represents the drive current of the first synchronous rectifier device 108. IG2 shown in part (h) of FIG. 10 represents a drive current of the second synchronous rectifier device 109.
Next, the operation of the conventional switching power supply configured as mentioned above will be described.
When the first PWM signal VG1 from the control circuit 114 is driven high (high level) at time T0 shown in FIG. 10, the first switching device 103 and the fourth switching device 106 are simultaneously turned ON. When the first switching device 103 and the fourth switching device 106 are turned ON as described above, an input voltage is applied to the primary winding 107a of the transformer 107 and a voltage is generated in the secondary winding 107b of the transformer 107. At this time, a voltage is also applied to the primary winding 121a of the drive transformer 121, and a positive voltage is generated in the first secondary winding 121b of the drive transformer 121, while a negative voltage is generated in the second secondary winding 121c of the drive transformer 121. As a result, the output of the first reverse goes low (low level), while the output of the second reverse remains high (high level), and the first synchronous rectifier device 108 is turned OFF, while the second synchronous rectifier device 109 is in the ON state.
The voltage generated in the first secondary winding 107b of the transformer 107 is applied via the second synchronous rectifier device 109 in the ON state to the smoothing circuit comprising the inductance device 110 and the smoothing capacitor 111.
When the first PWM signal VG1 is driven low at time T1, the first switching device 103 and the fourth switching device 106 are turned OFF. At this time, the voltage applied to the primary winding 121a of the drive transformer 121 becomes zero, and the voltages generated in the first secondary winding 121b and the second secondary winding 121c of the drive transformer 121 also become zero. As a result, the first reverse and the second reverse are both high, and the first synchronous rectifier device 108 and the second synchronous rectifier device 109 are simultaneously in the OFF state. The current flowing through the inductance device 110 divides to flow through the first secondary winding 107b and the second secondary winding 107c of the transformer 107 via the first synchronous rectifier device 108 and the second synchronous rectifier device 109 in the ON state. As a result, the exciting current of the transformer 107 becomes continuous. At this time, the voltages generated in the first secondary winding 107b and the second secondary winding 107c of the transformer 107 become zero, so that the voltage applied to the smoothing circuit (110, 111) also becomes zero.
When the second PWM signal VG2 from the control circuit 114 is driven high (high level) at time T2, the second switching device 104 and the third switching device 105 are simultaneously turned ON. When the second switching device 104 and the third switching device 105 are simultaneously turned ON as described above, an input voltage is applied to the primary winding 107a of the transformer 107 in the direction opposite to that at time T0. At this time, a voltage of the opposite polarity is also applied to the drive transformer 121, and the first synchronous rectifier device 108 is turned ON by the first reverse, while the second synchronous rectifier device 109 is turned OFF by the second reverse. As a result, the voltage generated in the second secondary winding 107c of the transformer 107 is applied via the first synchronous rectifier device 108 to the smoothing circuit (110, 111).
In the conventional switching power supply, by the operation mentioned above, the time of the voltage applied to the smoothing circuit is changed according to the ON/OFF ratios of the first PWM signal VG1 and the second PWM signal VG2, and the output voltage can be thereby adjusted. The output voltage is detected by the first detection resistance 150 and the second detection resistance 151, compared with the voltage of the reference power supply 152 and amplified at the differential amplifier 153, and input to the photocoupler 154. The control circuit 114 generates and outputs a PWM signal according to the output signal from the photocoupler 154. In the conventional switching power supply, as mentioned above, a negative feedback circuit is comprised of the control circuit 114, and the output voltage is stabilized.
However, with the configuration of the conventional switching power supply configured as mentioned above, when the synchronous rectifier device is driven, at the turn-on time, the drive current is supplied from the output voltage via the driving circuit, and the gate charge is charged. At the turn-off time, the gate charge is short-circuit discharged via the driving circuit. Therefore, there is a problem that, when the synchronous rectifier device is ON/OFF-driven, a rectification loss is produced. To reduce the conduction loss of the synchronous rectifier device, it is desirable to use a synchronous rectifier device which is large in chip size. The synchronous rectifier device which is large in chip size has a problem that, since the gate capacitance is large and the drive loss is great, the rectification loss including this drive loss cannot be reduced. In the conventional switching power supply configured as mentioned above, there is a problem that, since the output voltage is used as a power supply of the driving circuit, the synchronous rectifier device cannot be driven when the output voltage is extremely low, for example, 1V.
The present invention intends to solve the problems in the conventional switching power supply, and is intended to provide a high-efficiency switching power supply which supplies a voltage with stability while charging and discharging the gate of a synchronous rectifier device without any loss, and can thereby exert fully the effect of the synchronous rectification.